Longitudinal structure of the vertical E x B drift and ion density seen from ROCSAT-1

We investigate the longitudinal distribution of the vertical E x B drift velocity and ion density in the low-latitude ionosphere using the first Republic of China Satellite (ROCSAT-1) data acquired during 1999-2004. The ROCSAT-1 observations during daytime demonstrate the presence of the longitudinally periodic patterns of the vertical E x B drift and plasma density on the topside F region (600 km). The four longitude sectors where the peaks in the plasma density are found are coincident with the peaks in the E x B drift. This observation may indicate the association of the large-scale longitudinal density structure with the daytime E-region dynamo electric field. The density structure exists before the occurrence of the pre-reversal enhancement (PRE) and therefore the PRE is not directly related to this phenomenon.